This invention relates generally to recording apparatus and more particularly to matrix printing systems capable of producing a printed recording or hard copy of a video frame appearing on a cathode ray tube display device.
When using a cathode ray tube display terminal in a computer system, a permanent record of a frame is often desirable. The exhibited data are in the form of binary bits controlling the unblanking of the electron beam that is being repeatedly scanned across the face of the CRT. Each display frame is thus formed of a matrix of fluorescent dots with each dot representing the presence of a binary bit. Source data are generally provided from a random access memory to permit regeneration of the frame as long as required.
Hard copy printers, because of their relative slowness, require a form of buffer storage into which the binary data are read from the data source and temporarily retained until the printer can complete the job much later. The frame data can be obtained either from the memory of the display device or directly from the video input to the CRT display.
Buffer storage arrangements for hard copy printers in the past have used two techniques to capture the bit data for the frame. Either character codes are transmitted from the display to a character generator for the printer and the information recorded, or each scan line is captured and recorded singly in succession. The former technique has the advantage of tying up the display unit for the minimum time.
However, this method requires a larger amount of buffer storage since more bits must be held in storage concurrently. The latter technique requires storage for only a single scan line of bits, but the printer must record each line before the next can be stored. Thus use of the display unit is delayed until all lines are sequentially printed. An example of this technique is the system shown in U.S. Pat. No. 4,158,203 and the printing device must be equal in length to a full scan line, usually having several hundred elements.